It is more particularly in the pneumatics field that rigid or flexible, hose-like fluid power lines are frequently utilized as part of a releasable plug-in connection on fluid power joint elements, such as screw in or other type pipes fittings. In this respect, the joint element has a female socket part into whose end the fluid power line is inserted. In the inserted condition, the fluid power line is encircled by a resilient gripper sleeve, whose resilient gripping fingers act on the outer surface of the fluid power line to secure it in position. In order to seal off the plug-in connection, a sealing ring is arranged in the female socket, which on the one hand acts on an annular and generally cylindrical seal surface of the joint element and on the other hand surrounds the fluid power line coaxially with a sealing action.
A sealing ring, of the type initially mentioned, used for such purposes is described in European patent publication 0 379 655 A2. In its operational position it is arranged axially between an annular shoulder in the female socket and the gripping fingers of a gripper sleeve. On an axially inwardly facing side facing in the direction of insertion, two sealing lips form a V-like configuration which function as overpressure sealing lips, that is to say are responsible for a sealing action when there is an overpressure in the fluid power line. Since the seal is arranged in an axially sliding manner in the female socket, it is shifted towards the insertion end when there is an overpressure in the fluid power line, a thrust part provided or the axially outwardly facing side then cooperating with the gripping elements and thrusting the same additionally towards the periphery of the inserted fluid power line in order to enhance the gripping effect.
It has now been found that the during operation under vacuum, in which the pressure occurring in the fluid power line is less than the pressure of the surroundings, the known sealing ring does not provide a reliable sealing action under all conditions and, more particularly, when there is a high degree of vacuum, there may be entry of air from outside, something to be avoided under all circumstances.